A majority of modern motor vehicles employ internal combustion engines for propulsion. As a by-product of generating power, an internal combustion engine generates a stream of exhaust gas and also gives off heat energy. Accordingly, after a cold-start of the engine, i.e., when the engine is activated with its temperature at or near ambient, the engine proceeds through a “warm-up” period during which the engine's operating temperature is steadily increased.
During the first couple minutes after starting an internal combustion engine that has been started from cold, an amount of exhaust emissions can be significantly higher than emissions during the engine's steady state operation. In cold engines fuel does not vaporize completely, thus requiring richer air-fuel ratios. Rich air-fuel ratios, in turn, generate higher emissions of hydrocarbons, nitrogen oxides, and carbon monoxide, which diminish only as the engine reaches operating temperature. Cold engines also generate increased emissions because catalytic converters are less efficient under cold conditions and until they reach their operating temperature.
As the engine is started from cold and proceeds through the “warm-up” period, a passenger cabin of the host motor vehicle can also take time to warm-up from ambient temperature. Cabin warm-up in motor vehicles is frequently accomplished by a system capturing the engine's by-product heat energy via engine coolant, and then circulating the engine coolant through a heat-exchanger to transfer the heat energy to air forced into the vehicle cabin. Accordingly, in such systems, cabin warm-up is influenced by the rate of engine warm-up.